Wide band-gap (WBG) power semiconductor devices are approaching the point of commercial adoption in some power electronic converter applications. They have a larger band-gap than silicon, giving rise to an increased breakdown electric field strength and an increased maximum junction temperature.
The increased electric field strength results in a narrower drift region for a given breakdown voltage; advantages this may give rise to are, e.g., (i) lower on-state voltage drop (resistance for normalized area) and thus lower conduction losses; and/or (ii) faster switching and thus lower switching losses. Increased operating junction temperature is also a potential benefit, although existing device packaging cannot withstand the severe temperature cycling resulting from this under typical variable load conditions.
While the reduced conduction losses may be of most immediate benefit, greatly increased switching speed—and hence reduced switching losses and/or increased power converter switching frequencies—may generally only be taken advantage of if the commutating inductance is greatly reduced from typical levels found in IGBT-based converters. This may be in the region of 30 nH for a low-voltage converter, e.g., 690 V ac supply, using power devices with breakdown voltages of, e.g., 1200 or 1700 V. Indeed, even existing IGBTs sometimes have to be slowed down to reduce the inductive voltage overshoot levels sufficiently; directly replacing these with WBG devices (e.g., SiC MOSFETs of the same breakdown voltage rating) would generally require the same switching speed to be achieved, this may result in a significant increase of the WBG device switching losses that may make their adoption pointless.
To take advantage of WBG device potential, it may be considered to develop a switching circuit with a very small commutation inductance, preferably less than a few nH, to allow fast switching to take place without resulting in large voltage overshoots. The side effect of this however may be very fast di/dt and dv/dt switching edges, the latter also being an issue for adoption in motor drives. Furthermore, the very small commutation inductance may simply not achievable in existing converter designs above a few kW, due to the highly compact layout required.
Hence if WBG devices are to be adopted in high power converters, e.g., 100 kW and above, the ability to cope with existing commutation inductances and deliver an apparent dv/dt in line with existing IGBT switching is desirable. Furthermore, to aid market adoption the devices would preferably be available in a package similar to those used currently—e.g., EconoDual/Pack™, PrimePack™, HPM—to avoid having to completely start from scratch in converter design.
The field of power converters continues to provide a need for a switching topology that may for example allow, inter alia, greater energy efficiency, improved reliability, lower cost, compact design, suitability for standard power converter packaging, fast and/or low switching loss operation (for example in the presence of relatively large inductances to external capacitors), increased power converter switching frequencies, lower conduction losses, high maximum operating temperature, high DC supply voltage, for example relative to the power switching device breakdown voltage, reduced voltage overshoot, improved protection of inductive loads having windings (e.g., motors), etc.